Digital pre-distortion is an effective technique compensative for the non-linear behavior of a transmitter in a wireless communication device. One important step in digital pre-distortion is characterizing the non-linearity of the transmitter, i.e., determining parameters that accurately describe the non-linear curve of the transmitter, from which digital pre-distortion parameters are derived. The non-linearity of the transmitter can be observed by detecting the output signal of the transmitter to the extent that the non-linearity is reflected in the signal quality of the transmitter output signal.
There are several methods for observing the non-linearity of a transmitter. A first method involves observing the in-band error vector magnitude (EVM) of the transmitter output signal. The non-linear characteristics of the transmitter are reflected in the in-band EVM degradation. A problem with this method is that the signal distortion due to the transmitter non-linearity occurs in the time domain, resulting in amplitude modulation (AM)-AM and AM-phase modulation (PM) distortion. The EVM needs be computed in the time domain in order to correlate the transmitter non-linearity and EVM degradation. Consequently, this method is not feasible for orthogonal frequency division multiple access (OFDMA) or code division multiple access (CDMA) systems where the symbol quality is not readily observed in the time domain. In an OFDMA system, the time domain waveform is converted to a frequency domain waveform from which the symbols are then extracted at the destination device. In a CDMA system, the time domain waveform is de-spread using code correlation techniques, and the symbols are extracted from the de-spread signals at the destination device.
Another method for observing transmitter non-linearity is to observe the out-of-band emissions of transmitter output signal, that is, energy outside the bandwidth of the transmit signal itself. This method relies on the fact that the non-linear characteristics of the transmitter will result in an increase in the out-of-band emissions, known as so-called “spectrum growth”. However, in order to implement this technique, the bandwidth of the feedback path from the transmitter output needs to be many times wider than the bandwidth of the transmitter output signal itself in order to observe 3rd and 5th order inter-modulation distortion. Furthermore, the dynamic range of the feedback path needs to be large enough in order to accurately observe higher order inter-modulation distortion. Further still, this method becomes problematic when the transmitter output power level fluctuates with traffic loading and downlink power control.
Accordingly, a better scheme is needed in order to characterize the non-linearity of a transmitter for use in connection with digital pre-distortion compensation techniques.